dt-bindings: net: ethernet: Update mt7622 docs and dts to reflect the new phylink API

This patch the removes the recently added mediatek,physpeed property.
Use the fixed-link property speed = <2500> to set the phy in 2.5Gbit.
See mt7622-bananapi-bpi-r64.dts for a working example.

Signed-off-by: René van Dorst <opensource@vdorst.com>
Signed-off-by: David S. Miller <davem@davemloft.net>